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; Copyright (c) 1993-2008 by Richard Kelsey and Jonathan Rees. See file COPYING.
; Queues
(define (make-queue)
(cons '() '()))
(define (queue-empty? q)
(and (null? (car q))
(null? (cdr q))))
(define (enqueue! q obj)
(set-car! q (cons obj (car q))))
(define (dequeue! q)
(normalize-queue! q)
(let ((head (car (cdr q))))
(set-cdr! q (cdr (cdr q)))
head))
(define (normalize-queue! q)
(if (null? (cdr q))
(begin (set-cdr! q (reverse (car q)))
(set-car! q '()))))
(define (queue-head q)
(normalize-queue! q)
(car (cdr q)))
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